Melanins are widespread in nature, providing pigmentation, photoprotection, anti-oxidant, metal binding and other biological properties. An important subclass of melanins is eumelanin, which forms from tyrosine through a pathway involving 3,4-dihydroxyphenylalanine (DOPA) oxidation, intramolecular cyclization, oligomerization, and aggregation to form an insoluble and heterogeneous solid. Research on eumelanins is in part motivated by an interest in understanding structure-property relationships in the context of biological function, but also due to a growing interest in the interesting optical, electrical properties and technological applications of melanin thin films.
Synthetic and natural melanins are insoluble in many solvents and this property represents a challenge for processing into useful forms such as thin films. In the past, the deposition of melanin-like thin films on substrates has been accomplished by solution casting, (electro)spraying, spin coating, electrochemical deposition, and pulsed laser deposition. In addition to generally requiring the solubilization of melanin in organic solvent or aqueous NaOH or ammonia, most reported methods are either line-of-sight, require sophisticated equipment, employ complex multi-step protocols, or can only be performed on conducting substrates. Due to these limitations, there remains a significant need for simple and versatile approaches to melanin thin film deposition. Methods for forming melanin thin films that avoid the need for significant infrastructure and that can accommodate a variety of substrate compositions, shapes and configurations, will accelerate the development of practical applications for this interesting class of bioinspired materials.
A simple and versatile method to modify surfaces with melanin-like coatings involving dip-coating of substrates into an aqueous solution of catecholamine mimics of DOPA-rich mussel adhesive proteins, such as dopamine, is disclosed in U.S. Patent Publication No. 2008/0149566 (which is incorporated by reference in this document for all purposes). Dopamine undergoes spontaneous auto-oxidation in mildly basic and aerated aqueous solution to form an adherent polydopamine (PDA) film on virtually any substrate. PDA, also referred to as dopamine-melanin because of its chemical similarity to eumelanin, can then act as a primer for further modification leading to numerous applications such as biomolecule immobilization, surface energy modification, biomineralization and biosensing. Molecules of interest can also be co-deposited simultaneously with dopamine in a one-step reaction to create surfaces with desired properties.
Several derivatives of dopamine and other catecholamines polymerize in a similar fashion onto a wide variety of substrates. For example, norepinephrine was shown to polymerize into adherent films with the added ability of initiating ring-opening polymerization of ε-caprolactone due to the presence of a hydroxyl group not found on dopamine. It would be of interest to spontaneously polymerize DOPA-melanin (DM) films on a variety of substrates in a similar fashion as PDA, as this would further accelerate technological applications of eumelanin-like thin films. In comparison to PDA, DM films would be expected to exhibit a higher concentration of free carboxylic acids that can be exploited for a variety of applications.
However, historical reports of the alkaline auto-oxidation of DOPA in water or low ionic strength buffer resulted in a melanin-like product which is soluble or a solution-stable supramolecular nanoaggregate. Those in the art have experienced difficulty forming adherent films from DOPA using the standard conditions developed for spontaneous PDA film formation (buffered aerated H2O, pH 8-9). In-situ grown DM films have only been shown to achieve thicknesses of 10 nm or less under conditions that typically yield much thicker films using dopamine. Thus, compared to PDA, DM films are considerably more difficult to grow on substrates by spontaneous autoxidation of DOPA.
Accordingly, there is a need in the art for improved methods of producing DM films and particles for a variety of applications.